Deleting the same question multiple times gives an error

[halfwhole]

A question already exists with index 20. Then input this command: delete 20 20 20 20 20

INFO: ----------------[USER COMMAND][delete 20 20 20 20 20]
Exception in thread "JavaFX Application Thread" seedu.revision.model.answerable.exceptions.AnswerableNotFoundException
	at seedu.revision.model.answerable.UniqueAnswerableList.remove(UniqueAnswerableList.java:80)
	at seedu.revision.model.AddressBook.removeAnswerable(AddressBook.java:94)
	at seedu.revision.model.ModelManager.deleteAnswerable(ModelManager.java:100)
	at seedu.revision.logic.commands.main.DeleteCommand.execute(DeleteCommand.java:55)
	at seedu.revision.logic.MainLogicManager.execute(MainLogicManager.java:52)
	at seedu.revision.ui.MainWindow.executeCommand(MainWindow.java:128)
	at seedu.revision.ui.CommandBox.handleCommandEntered(CommandBox.java:59)
	at seedu.revision.ui.CommandBox.lambda$new$1(CommandBox.java:44)
	at com.sun.javafx.event.CompositeEventHandler$NormalEventFilterRecord.handleCapturingEvent(CompositeEventHandler.java:282)
	at com.sun.javafx.event.CompositeEventHandler.dispatchCapturingEvent(CompositeEventHandler.java:98)
	at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:223)
	at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:180)
	at com.sun.javafx.event.CompositeEventDispatcher.dispatchCapturingEvent(CompositeEventDispatcher.java:43)
	at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:52)
	at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
	at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
	at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
	at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
	at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
	at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
	at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
	at javafx.event.Event.fireEvent(Event.java:198)
	at javafx.scene.Scene$KeyHandler.process(Scene.java:4058)
	at javafx.scene.Scene$KeyHandler.access$1500(Scene.java:4004)
	at javafx.scene.Scene.processKeyEvent(Scene.java:2121)
	at javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2595)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:217)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:149)
	at java.base/java.security.AccessController.doPrivileged(Native Method)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleKeyEvent$1(GlassViewEventHandler.java:248)
	at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:390)
	at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:247)
	at com.sun.glass.ui.View.handleKeyEvent(View.java:547)
	at com.sun.glass.ui.View.notifyKey(View.java:971)
	at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
	at com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
	at java.base/java.lang.Thread.run(Thread.java:834)